The highly ordered connectivity of axons and dendrites ("wiring") requires an immensely complex developmental program that is still poorly understood. Many studies in the field of developmental neurobiology have shown that the wiring of the brain during development are the central cause of many disorders of the nervous system, including epilepsy, schizophrenia, autism, and mental retardation. Understanding the basic evolutionary conserved mechanisms of wiring specificity will help in identifying the crucial molecular components that may be altered in the development of human neurological diseases. Specifically, this proposal aims to study the functional importance of the Drosophila cell adhesion receptor Dscam (named after the homologous human protein Down Syndrome Cell Adhesion Molecule, DSCAM). Several studies have characterized Dscam as an important cell surface receptor expressed and required in the Drosophila nervous system as well as the immune system. The Dscam gene is highly complex with tandem arrays of alternative exons 4, 6, 9, and 17 allowing for the generations of maximally 38,016 isoforms. In vitro studies suggest that homophilic binding specificity of Dscam isoforms may allow for an unprecedented level of recognition specificity within the nervous system. Our most recent results provide the first direct evidence that the large repertoire of Dscam isoforms is indeed required for the specificity and precision of axonal targeting within the central nervous system (CMS). We propose to use the model organism Drosophila to study fundamental mechanisms by which specific surface receptors control the developmental wiring process of complex neuronal circuits. We will use the Dscam gene as an experimental paradigm to examine the specificity and importance of highly diverse neuronal surface receptors. To this end we will examine the cellular and subcellular expression of different Dscam isoforms using RT-PCR, isoform specific antibodies as well as genetically encoded protein tags. We will use genetic manipulations and transgenic flies to test the hypothesis that Dscam isoforms are utilized in an instructive way to control specific axonal connections within the CNS. We will investigate whether Dscam- Dscam interactions control axonal targeting in vivo by engineering transgenes, which will be utilized to genetically manipulate Dscam expression or diversity. Furthermore, we intend to dissect the molecular mechanisms of Dscam adhesion or signal transduction mechanisms. The overall goal of our studies is to contribute to a better understanding of how structurally diverse surface receptors specify and control the precision of neuronal connectivity.